Method of improving photomask geometry

ABSTRACT

Photomask manufacture is improved by adding assist bars  320, 340  for isolated features  300.  The bars  320, 340  are added at a center to center spacing that corresponds to the center to center spacing for densely packed features. By matching the assist bars to the densely packed features, the combined diffraction pattern of the isolated features is modified to more closely resemble the diffraction pattern of the densely packed features.

RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. Patent ApplicationSerial No. 60/182,367 filed Feb. 14, 2000 and No. 60/185,046 filed Feb.25, 2000.

FIELD OF INVENTION

[0002] The present invention relates to the field of microlithographyfor processing of integrated circuit devices, particularlyphotolithographic masking and imaging.

BACKGROUND

[0003] Methods of proximity effect reduction have been introduced whichare comprised of additional lines, sometimes referred to as intensityleveling bars, into a mask pattern. This is disclosed by J. F. Chen inU.S. Pat. No. 5,242,770, and its entire disclosure is incorporated byreference in this application. These leveling bars perform the functionof matching edge intensity gradients of isolated features on the mask tomore dense features on the mask, as shown in FIG. 1. In this figure,lines 30-34 are all designed to the same dimension and spacing. Thepatterning is such that line 32 experiences three separate proximitysituations, one that is closely packed to lines 30, 31, 33, and 34, onethat is isolated, and one that is surrounded by intensity leveling bars,36. In the preferred embodiment of U.S. Pat. No. 5,242,770, the width ofthe leveling bars, 36, is equal to one fifth of the critical dimensionfor the line and the separation of leveling bar 36 from the line 32, asdefined by the edge to edge distance (such as the separation distance 14shown in FIG. 2) to be 1.1 times the critical dimension of the line 32.This distance is preferred for a dark line in a clear field. A distanceequal to the critical dimension is preferred for a clear line in a darkfield.

SUMMARY OF THE INVENTION

[0004] I have discovered that the existing methods described todetermine the optimum leveling bar separation and feature size, such asthat described U.S. Pat. No. 5,242,770, do not lead to an optimumcondition. I have also discovered a method of determining the optimumseparation and leveling bar width that is based on the frequencydistribution of the masking features. By using this method, the matchingof the performance of isolated features and dense features is madepossible.

[0005] An improvement for determining the placement of sub-resolutionimaging assisting bars (referred to as leveling bars in U.S. Pat. No.5,242,770) to reduce the proximity effects when imaging isolated anddense masking features for integrated circuit microlithography isdescribed. According to the present invention, the placement ofadditional masking features is determined by considering the diffractionpatterns in the lens pupil of the primary feature of interest underconditions where the feature is closely packed with other similarfeatures (referred to as dense geometry) and also where the feature isisolated or nearly isolated, where the next closest feature is separatedon the order of more than three times the feature size. The differencesbetween these two diffraction patterns is generally the presence offirst diffraction orders for feature sizes near 0.5×Lambda/NA wherelambda is the exposing wavelength and NA is the lens numerical aperture.A pair of sub-resolution assisting bars placed on either size of theisolated feature is chosen so that their frequency distribution in thelens is cosinusoidal with the first node of the cosine above a frequencyof zero coincides with the frequency of the first diffraction order forthe dense features. As this cosine function is added to the diffractionpattern for the isolate feature, the diffraction pattern more closelyresembles that of the dense features, reducing proximity effects. Theseparation distance therefore depends on the pitch of the dense geometrythat image performance of the isolated geometry is targeted to. Toproduce a cosine of appropriate frequency, the center-to-centerseparation between the main isolated feature of interest and theassisting bar should be equal to the pitch of the desired densegeometry. The critical separation is the center-to-center spacing of themain feature and the assisting bar, not the edge to edge spacing used byothers. This is an important aspect of the invention. Thiscenter-to-center spacing value is twice the line dimension for 1:1 dutyratio features and greater than that for duty ratios greater than 1:1.The distance is equivalent to the desired pitch distance of the targeteddense features. To match the performance of 1:1.5 features, forinstance, the separation is 2.5× the line dimension.

[0006] The width of the assisting bars is determined by considering theeffect that a pair of non-zero width features has on the resultingcosine diffraction function in the lens pupil. For hypotheticalzero-width bars, the cosine exists in the lens pupil unmodulated and thefirst cosine nodes add to the isolated feature diffraction pattern toproduce the addition of equivalent first diffraction order energy. Asthe assisting bar width increases, the cosine function in the lens pupilis multiplied by the Fourier Transform of a rectangular functionrepresenting the bar width. This is a sinc function, which is a maximumat lowest frequencies at the center of the lens pupil and decreases athigher frequencies toward the outside of the lens pupil. The scalingproperties of the Fourier Transform are such that larger assisting barsproduce more dampening at lower frequencies. The impact is the reductionof the first nodes of the cosine function, reducing the effectiveness ofthe correction effect and also increasing the likelihood that the assistbars will print. The assist bar width should therefore be as small aspossible and as much below the incoherent diffraction limit(0.25×lambda/NA) as possible.

[0007] To compensate for the modulation of the cosine function producedin the lens pupil by the pair of non-zero width assisting bars, spatialfrequency filtering can be utilized. A filter that resembles thereciprocal of the modulating sinc function is desired. This spatialfrequency filter will result in a boosting of the first nodes of thecosine function and result in a closer resemblance between the densefeatures and the assist bar assisted isolated features.

DRAWINGS

[0008]FIG. 1 is mask test pattern utilizing edge intensity leveling baras shown in U.S. Pat. No. 5,242,770.

[0009]FIG. 2 is a mask test pattern showing isolated and densely packedfeatures.

[0010]FIG. 3. is a mask pattern of an isolated feature with edgeintensity leveling bars spaced in accordance with this invention.

[0011]FIG. 4. is a graphical representation of the diffraction patternof dense features of FIGS. 1 and 2 as a function of spatial frequency

[0012]FIG. 5 is a graphical representation of the diffraction patternfor the isolated features of FIGS. 1 and 2 as a function of spatialfrequency.

[0013]FIG. 6 is a graphical representation showing the intensity of themask signal of the assisting bars on opposite sides of the isolatedfeature of FIG. 3.

[0014]FIG. 7 is a graphical representation of the diffraction patternfor the assisting bars of FIG. 3 as a function of spatial frequency.

[0015]FIG. 8 is a graphical representation of the diffraction energydistribution for the isolated feature and its assisting bars of FIG. 3as a function of spatial frequency.

[0016]FIG. 9 is a graphical representation of the damping due to theassisting bar cosine diffraction energy.

[0017] FIGS. 10(a) and 10(b) compare off axis illumination without andwith assist bars on isolated features.

[0018] FIGS. 11(a)-11(d) are graphical representations of simulatedresults of diffraction patterns for (a) dense features, (b) an isolatedline, (c) an isolated line with one pair of assist bars, and (d) theassist bars themselves.

[0019] FIGS. 12(a)-12(d) are graphical representations of simulatedresults with two assists bar pairs for of diffraction patterns for (a)dense features, (b) an isolated line, (c) an isolated line with one pairof assist bars, and (d) the assist bars themselves.

[0020] FIGS. 13(a)-13(d) are graphical representations of diffractionorders for coherent illumination in a lens pupil at duty rations of 1:1,13(a), 1:3, 13(b); 1:5, 13(c) and 1:7, 13(d).

[0021] FIGS. 14(a) and 14(b) show the effects of diffraction energywithout 14(a) and with 14(b) one pair of assist bars.

[0022]FIG. 15(a) shows the results of redistributing diffraction energywith one pair of bars and FIG. 15(b) shows similar effects with twopairs of bars.

[0023] FIGS. 16(a)-16(d) show defocus of isolated features with noassist bars 16(a), with one pair 16(b), with two pair 16(c) and withthree pair 16(d); FIG. 16(e) shows the spacing of the assist pairs as sfunction of wavelength and numerical aperture.

DETAILED DESCRIPTION

[0024]FIG. 3 shows the preferred approach to designing the placement ofthe assist features on a mask. This example shows dark lines on a clearmask but a similar analysis can be performed for clear lines on a darkmask. The desired separation distance between the assist features 320,340 and the main feature 300 depends on the duty ratio that performanceis targeted to match. For a 1:1 duty, the separation distance is setequal to the pitch. The size of the assist feature does not impact itsplacement. Placing the assist feature at a multiple of pitch distancewill also show some results. As a practical matter the assist bars orfeatures are placed at a separation of desired pitch match. The assistfeature is made a small as possible. One may use a pupil filter withsinc (w/p) attenuation in the center vs. edge for inverse gaussianperformance. In the preferred embodiment, the pitch from center tocenter of the isolated feature and the assisting features is the same asthe center to center pitch of the dense features.

[0025]FIG. 4 shows the diffraction pattern vs. spatial frequency fordense features and FIG. 5 shows the diffraction pattern vs. spatialfrequency for an isolated feature. FIG. 6 shows assist bars placed onopposite sides of the isolated feature, as shown in FIG. 3. Thediffraction pattern for the assist features alone is shown in FIG. 7.The frequency of the cosine function should be cos(2πpu) where p is thepitch of the dense features targeted to match and u is the onedimensional frequency coordinate in the lens pupil. As seen from FIG. 7,the cosine function is modulated by a sinc function, a result of thenon-zero width of the assist bars. The frequency of the cosine is chosenso that the first nodes coincide with the first diffraction order of thedense features. This occurs when the assist bars are spaced at acenter-center distance of the pitch distance of the desired dense lines.The resulting diffraction energy distribution is shown in FIG. 8. Thepattern of assisted isolated feature of FIG. 8 is closer to the densepattern of FIG. 4 than is the pattern of the isolated feature of FIG. 5.For small features (close to 0.5×lambda/NA), only a small portion ofthis diffraction energy falls within the lens. More specifically, energynear and below the frequency of the first diffraction order for thedense lines (and near and below the first node of the assisting barsdiffraction energy cosine) is collected.

[0026] An enveloping sinc function modulates the assisting bar cosinediffraction energy and causes a dampening effect to the first cosinenodes. FIG. 9 shows this effect. The loss in energy at the first nodalor diffraction order frequency is exactly sinc(w/p)=[sin(πw/p)]/[πw/p]for a assisting bar width of w and a separation distance (equal to thedense pitch) of p.

[0027] As an example consider FIG. 8 where a 260 nm dense pitch maskfeature (130 nm 1:1) and target isolated proximity effect correctionusing assisting bars. An assisting bar width of and 80 nm is chosen,which may represent the current limitations to mask making. Themodulation of the first cosine nodes (equivalent to the firstdiffraction order) is calculated as sinc(w/p)=0.85 or 85% attenuation ascompared to the central pupil amplitude as can be seen in FIG. 8.Smaller assist features would result in less attenuation but thesebecome challenging to fabricate. Therefore, use of pupil filtering of atleast 85% attenuation of the center of the pupil as compared to thefirst diffraction order position is desirable for these assist features.This can be accomplished using spatial frequency filtering in the pupilor in other equivalent pupil positions or Fourier Transform planes inoptical system. The filter is designed with a transmission vs. frequencydistribution designed to resemble 1/[sinc(w/p)] over the range of thelens NA (which has a frequency of NA/lambda).

[0028]FIG. 11 shows simulated results using a commercial lithographysimulator (Prolith/2 version 6.05d) for 150 nm geometry using 193 nm.The diffraction pattern is an isolated feature at nearly 1:8 spacing isshown in FIG. 11b; the diffraction pattern for dense features of about1:1 is shown in FIG. 11a. The diffraction pattern for assisting barswith a 75 nm width and a center-to-center separation of 300 nm is shownin FIG. 11(d). The diffraction pattern for the assisted 150 nm featuresof FIG. 11(c) more closely resembles that of the dense features of FIG.11(a) than does the pattern of the isolated feature without assistingbars, FIG. 11(b).

[0029] It is further possible to improve matching the diffractionenergies between isolated and dense features by adding a second pair ofassisting features, centered and each separated by a distance-to-centerof twice the desired pitch from the center feature of interest. This hasthe impact of adding additional cosine nodes at a frequency positioncorresponding to the desired first diffraction order and also at afrequency position of half of the desired first order, equivalent to0.5/p. This results in boosting energy at the first diffraction orderfrequency and reducing the amplitude of the zero order. FIG. 12 showsthe results for 150 nm geometry and two pairs of 50 nm assisting bars.These bars are spaced at 300 and 600 nm from the center of the mainfeature to the center of the assisting bar. Again, the pattern of theisolated feature with assisting bars as shown in FIG. 12(c) more closelyresembles the diffraction pattern of the dense features in FIG. 12(a)than does either the isolated feature with a single pair of bars, FIG.12(b) or the pattern of the double bars, FIG. 12(d).

[0030] Preferred Duty Ratio

[0031] Imaging performance is more closely matched to dense features byadding assisting bars with predetermined separation and sizingparameters in proximity to isolated features. Optimum separation andsizing parameters of the bars are described in this invention wherecosinusoidal functions are controlled within then lens pupil. Sincefeatures are realistically never completely isolated (there is alwayssome neighboring geometry if a large enough window is considered), thiscan be taken into account when designing mask feature and assistingfeature layout.

[0032] Let us consider semi-isolated features compared to densely packedfeatures. FIGS. 13(a)-13(d) show a situation where 130 nm lines arebeing imaged at a 193 nm exposure wavelength. The figure shows thediffraction orders for coherent illumination in a lens pupil with aneffective numerical aperture of unity. Duty ratio values of 1:1, 1:3,1:5, and 1:7 are shown where the 2^(nd) diffraction order of the 1:3duty ratio features coincide with the 1^(st) diffraction order for the1:1 duty ratio. For 1:5 duty ratio features, the 3^(rd) order coincideswith the 1:1 1^(st) order and for the 1:7 duty ratio features, the4^(th) order coincides. This is an important situation because anassisting feature cosinusoidal frequency functions exist within the lenspupil and their impact is greatest if maximum diffraction energy can beplaced at the first diffraction order position for features of targetedpitch. In all of these cases, there is diffraction order energy at theserequired frequency positions. Because of the features are not completelyisolated, assisting features added to the more isolated geometry(defined as 1:3 and greater) also have a repeating nature. The cosinefunctions in the pupil that result from this periodicity are alsosampled at these same frequency, which can be represented as

[Cos(2πpu)×comb(up′)]×sinc(w/p).

[0033] Where the comb function is a series of Dirac delta functions (orimpulse functions), u is the frequency value in the pupil, p′ is thepitch of the isolated feature. If p′ is a multiple of p (the pitch ofthe desired matching dense features), there will be a coincidence ofdiffraction orders, as described in the example above and in FIG. 13.This is an important aspect of the invention. During design of maskfeatures, the performance of assisting bars can be improved when asemi-isolated feature (not completely isolated but with distantneighboring geometry) has a pitch value which is a multiple of the pitchvalue of the targeted feature. For 1:1 dense targeted features, thisbecomes the pitch equivalent to 1:3, 1:5, 1:7 and so forth for the moreisolated features. If the pitch of the semi-isolated features was 1:2 or1:4 the pitch of the dense features, there would be little or noincrease in proximity correction. For 1:1.2 dense targeted features,this becomes 1:3.4, 1:5.6 and so forth. For 1:1.5 dense targetedfeatures, this becomes 1:4, 1:6.5 and so forth. Designing mask geometryto contain these feature duty ratio or pitch values will enhance theperformance improvement achieved when using assisting feature proximitycorrection methods.

[0034] The effects of partial coherence can lead to further improvement.By increasing the size of the effective source though larger partialcoherence values, diffraction energy of the dense and assisted isolatedfeatures is brought closer together. This does not impact the choice ofassisting feature position (separation) or size.

[0035] Globally Isolated Features

[0036] I have discovered that when features are very isolated (generallygreater than 1:7 for lines or 7:1 for spaces), the method for placementof assist features can be modified somewhat. The goal is to reduce thesensitivity of isolated geometry (lines or spaces) to aberrationeffects, which includes defocus. In a perfect lens system, maximum depthof focus is the greatest concern since no lens aberration would exist.In the presence of aberrations, both maximum depth of focus and minimumimpact of aberrations is desired. I have shown in previous papers(Smith, SPIE Optical Microlithography XI and XII 1998 and 1999) that theimpact of aberrations is dependant on the position of diffraction energywithin the pupil. By using assisting features, diffraction energy of afeature can be redistributed to lie within the most advantageousportions of the pupil. To maximize depth of focus (and minimize theimpact of defocus), it is desirable to have maximum diffraction energyat the edge of the pupil with as little energy in the center of thepupil as possible (image degradation is caused by phase error ofdiffraction orders across the pupil). The same situation exists forspherical and other symmetrical aberrations. For astigmatism, thesituation is such that distribution of energy toward the edge of thepupil may lead to an increase in image degradation across horizontal andvertical directions. Coma effects can be minimized when balanced withtilt as energy is distributed toward the edge of the pupil.

[0037] Isolated features experience more aberration that dense features.If assisting optical proximity correction bar pairs are placed on sidesof an isolated feature, the cosine “node” described earlier placediffraction energy at higher frequency positions in the pupil. If thereis no goal of matching a desired pitch (of more dense features) then thegoal should be to improve performance of the isolated features in thepresence of aberration and defocus. Aberration levels of today's lensesapproached 0.03 waves RMS (OPD). The amount of defocus introduced whenimaging over topography in a photoresist materials in much greater thanthis, on the order of 0.25 waves. In this case, it is best to optimizethe placement of assisting bars so that the defocus aberration isminimized. In the presence of other aberrations, any defocus-aberrationinteractions may require design so that energy is placed at othersimilar locations but for the most part, the defocus condition willover-ride. (The above references describe optimum pupil positions forminimizing aberration).

[0038] In order to place diffraction energy at the edge of the pupil,the frequency of the pupil edge must be known. For a given numericalaperture (NA) and wavelength (λ), this frequency is NA/λ. The assistingbars should be placed at multiples of this NA/λ frequency. By placing apair of bars with each spaced at a distance λ/NA from the main feature(center to center), the fundamental frequency of the primary cosine andharmonics of higher frequency cosines adds diffraction energy at theoptimum pupil edge position. One pair of bars at a separation of λ/NAcan be used. Two pairs of bars at the first separation distance and at2λ/NA can be used for improvement. Three pairs improve further by addingan additional pair at 3λ/NA. Four or more pairs also improves. The limitis the point where minimal additional improvement is realized or maskgeometry inhibits additional pairs.

[0039]FIG. 14(a) shows an application for 248 nm, 0.52NA and 180 nm 21:1spaces (very isolated). By adding assist space features 60 nm wide at aseparation of 400 nm, the diffraction energy in the pupil is movedtoward the edge and away from the center. See FIG. 14(b). FIGS. 15(a)and 15(b) show other examples where 2 and 3 pairs of bars providefurther improvements.

[0040] FIGS. 16(a)-(e) show how the invention will improve performancewith defocus aberration. FIG. 16(e) shows a mask pattern with threepairs of assist bars. A defocus value of 0.05 waves was introduced tocompare an unassisted bar with 1, 2 and three pairs of bars. The RMS OPDfor the isolated spaces is 0.0278 waves. With 1 pair of assist features,FIG. 16(b) shows the value decreases to 0.0235 waves. With 2 pairs thevalue decreases to 0.0203 waves. See FIG. 16(c). With 3 pairs, as seenin FIG. 16(d), the value decreases to 0.0164 waves. This dramaticimprovement will lead to a decrease in the impact of defocus to imaginga 180 nm space. Assist features of 60 nm in width do not print in asuitably optimizing thresholding resist process. The depth of focus cantherefore be improved.

[0041] Off-Axis Illumination

[0042] I have also discovered that OPC methods can be optimized forparticular conditions of customized or off-axis illumination. Thecontrol of the relative size of the illumination system numericalaperture has historically been used to optimize the performance of alithographic projection tool. Control of this NA with respect to theprojection systems objective lens NA allows for modification of spatialcoherence at the mask plane, commonly referred to partial coherence.This is accomplished through specification of the condenser lens pupilsize with respect to the projection lens pupil in a Köhler illuminationsystem. Essentially, this allows for manipulation of the opticalprocessing of diffraction information. Optimization of the partialcoherence of a projection imaging system is conventionally accomplishedusing full circular illuminator apertures. By controlling thedistribution of diffraction information in the objective lens with theilluminator pupil size, maximum image modulation can be obtained.Illumination systems can be further refined by considering variations tofull circular illumination apertures. A system where illumination isobliquely incident on the mask at an angle so that the zeroth and firstdiffraction orders are distributed on alternative sides of the opticalaxis may allow for improvements. Such an approach is generally referredto as off-axis illumination. The resulting two diffraction orders can besufficient for imaging. The minimum pitch resolution possible for thisoblique condition of partially coherent illumination is 0.5λ/NA, onehalf that possible for conventional illumination. This is accomplishedby limiting illumination to two narrow beams, distributed at selectedangles. The illumination angle is chosen uniquely for a given wavelength(λ), numerical aperture (NA), and feature pitch (d) and can becalculated for dense features as sin⁻¹(0.5λ/d) for NA=0.5λ/d. The mostsignificant impact of off axis illumination is realized when consideringfocal depth. In this case, the zeroth and 1st diffraction orders travelan identical path length regardless of the defocus amount. Theconsequence is a depth of focus that is effectively infinite.

[0043] In practice, limiting illumination to allow for one narrow beamor pair of beams leads to zero intensity. Also, imaging is limited tofeatures oriented along one direction in an x-y plane. To overcome this,an annular or ring aperture has been employed that delivers illuminationat angles needed with a finite ring width to allow for some finiteintensity. The resulting focal depth is less than that for the idealcase, but improvement over a full circular aperture can be achieved. Formost integrated circuit application, features are limited to horizontaland vertical orientation, and a quadrupole configuration may be moresuitable. Here, poles are at diagonal positions oriented 45 degrees tohorizontal and vertical mask features. Each beam is off-axis to all maskfeatures, and minimal image degradation exists. Either the annular orthe quadrupole off-axis system can be optimized for a specific featuresize, which would provide non-optimal illumination for all others. Forfeatures other than those that are targeted and optimized for, higherfrequency components do not overlap, and additional spatial frequencyartifacts are introduced. This can lead to a possible degradation ofimaging performance.

[0044] When considering dense features (1:1 to 1:3 line to space dutyratio), modulation and focal depth improvement can be realized throughproper choice of illumination configuration and angle. For isolatedfeatures, however, discrete diffraction orders do not exist; instead acontinuous diffraction pattern is produced. Convolving such a frequencyrepresentation with either illumination poles or annular rings willresult in diffraction information distributed over a range of angles.Isolated line performance is, therefore, not improved with off-axisillumination. When features are not isolated but have low density (>1:3line to space duty ratio), the condition for optimum illumination willnot be optimal for more dense features. Furthermore, the use of off-axisillumination is generally not required for the large pitch values thathave a low-density geometry. As dense and mostly isolated features areconsidered together in a field, it follows that the impact of off-axisillumination on these features will differ, and a large disparity indense to isolated feature performance can result.

[0045] Optical proximity correction (OPC) using assist features can leadto improvement with off-axis illumination (OAI). By designing assistfeatures around the “preferred duty ratio” described earlier, and bydesigning off-axis illumination around this same duty ratio (see forinstance my patent application “Illumination Device for ProjectionSystem and Method for Fabricating, U.S. patent application Ser. No.09/422,398, whose disclosure is herein incorporated by reference), OPCand OAI can work together to reduce the process complexity of eitherapproach alone or make possible imaging resolution and DOF that couldnot be easily achieved otherwise. The concept is similar to thatdescribed earlier. By adding assisting bars with predeterminedseparation and sizing, higher diffraction orders can be positioned tooverlap lower diffraction orders in the objective lens pupil. Designingaround “mask feature harmonics” with both illumination and OPC allowsfor more manufacturable solutions for sub-half wavelength featureresolution.

[0046]FIG. 10(a) shows the results of off axis illumination on a patternwith isolated features and no assist bars. Compare it to FIG. 10(b)where assist bars are added. Off axis illumination has been used toilluminate 150 nm lines with a line spacing of 450 nm, using awavelength of 248 nm, a numerical aperture of 0.70, and dipole off axisillumination. The dipole illumination consists of two poles oriented onan axis orthogonal to the axis of the 150 nm long lines on thephotomask. The illumination angle of each pole in the illuminator issin⁻¹((lambda)/(2*p)) where p is a value of 300 nm, the pitch of equalline/space 150 nm features. FIG. 10(a) shows the distribution ofdiffraction orders in the objective lens pupil for the illumination of150 nm lines with a 450 nm pitch under these conditions. The energy inthe center of the pupil is 30% of the intensity of the diffractionorders at the edge of the pupil. This will result in image degradationas the image plane is varied through focus. By using assisting bars,FIG. 10(b) shows that the energy at the center of the pupil is reducedwhen using dipole off axis illumination. This decreases the imagedegradation as the image plane is varied through focus. The effect isshown for dipole illumination. Other off-axis illumination will givesimilar results.

1. In a photomask for optically transferring a lithographic patterncorresponding to an integrated circuit (IC) from said photomask onto asemiconductor substrate, said pattern including a plurality of denselyspaced features and an isolated feature spaced from the densely spacedfeatures, said isolated feature having at least one pair of assistingbars on opposite sides of said isolated feature and spaced equally fromthe corresponding side of the isolated feature opposite the assistingbar, wherein the assisting bars alter the edge intensity gradient ofedges of the isolated feature to reduce proximity effects, theimprovement comprising: spacing the assisting bars in accordance withthe pitch of the densely space features.
 2. The photomask of claim 1wherein the assisting bars are spaced so that a first node of theirdiffraction cosine falls at the first diffraction order position of thedensely spaced features.
 3. The photomask of claim 1 wherein theisolated feature is spaced from the densely spaced feature by at least 7times the pitch of the densely spaced features and the assisting barsare spaced at equal multiples of a distance of lambda/NA from the centerof the isolated feature, where lambda the illumination wavelength and NAis the numerical aperture.
 4. The photomask of claim 1 wherein theassisting bars are spaced from the isolated feature a distance forgenerating a diffraction pattern having a cosine function with nodesthat coincide with the first diffraction order of the densely spacedfeatures.
 5. The photomask of claim 1 wherein the width of eachassisting bar is as small as possible.
 6. The photomask of claim 1wherein the width of each assisting bar is less than the incoherentdiffraction limit.
 7. The photomask of claim 1 having two or more pairsof assisting bars where each additional pair is spaced a multiple of thepitch of the first pair.
 8. The photomask of claim 1 wherein theisolated and dense features are lines.
 9. The photomask of claim 1wherein the isolated and dense features are spaces.
 10. In aphotolithographic apparatus having an illuminator for directing lightonto a photosensitive surface of a semiconductor, the improvementcomprising a photomask for optically transferring a lithographic patterncorresponding to an integrated circuit (IC) from said mask onto asemiconductor substrate, said pattern including a plurality of denselyspaced features and an isolated feature spaced from the densely spacedfeatures, said isolated feature having at least one pair of assistingbars on opposite sides of said isolated feature and spaced equally fromthe corresponding side opposite the assisting bar, wherein the assistingbars alter the edge intensity gradient of edges of the isolated featureto reduce proximity effects wherein spacing the assisting bars inaccordance with the pitch of the densely space features and a spatialfrequency filter disposed between the source of light and the photomaskfor distributing diffraction energy.
 11. In a projection lithographyapparatus with a light source, an illumination stage for shaping thelight source, and a lens for directing the shaped light onto a photomaskfor projecting a pattern on the mask to a photosensitive surface of asemiconductor wafer, a method for locating optical proximity correctionfeatures on the photomask comprising: providing a set of densely packedmask features on the photomask, said densely spaced features having adense feature pitch; providing one of more isolated features on thephotomask; locating one or more assist features adjacent the isolatedfeatures a distance selected in accordance with the pitch of the densefeatures.
 12. The method of claim 11 wherein the assisting features aresub-resolution in size, are placed on both sides of the isolated featurea distance so that their frequency distribution in the lens iscosinusoidal with the first node of the cosine above a frequency of zerocoinciding with the frequency of the first diffraction order of thedense features.
 13. The method of claim 11 wherein the assistingfeatures have a width less than the incoherent diffraction limit of thelens.
 14. The method of claim 11 wherein the width is less than 0.25times the wavelength of the light source divided by the number apertureof the apparatus.
 15. The method of claim 11 further comprising the stepof spatially filtering the projected image of the photomask to attenuatethe center of the pupil of the apparatus.
 16. The method of claim 11further comprising a second set of assisting features located at twicethe pitch of the first set of assisting features.
 17. The method ofclaim 16 further comprising a third set of assisting features located atthree times the pitch of the first set of assisting features.
 18. Themethod of claim 11 further comprising m sets of assisting featureswherein each set is located at m times the pitch of the first set ofassisting features.
 19. In a projection lithography apparatus with alight source of known wavelength and a known numerical aperture, anillumination stage for shaping the light source, and a lens fordirecting the shaped light onto a photomask for projecting a pattern onthe mask to a photosensitive surface of a semiconductor wafer, a methodfor locating optical proximity correction features on the photomaskcomprising: providing a set of densely packed mask features on thephotomask, providing one of more isolated features on the photomask;locating one or more assist features adjacent the isolated features adistance selected in accordance with the wavelength and numericalaperture.
 20. The method of claim 20 wherein there are m sets ofassisting features and each set m of features is spaced a distancecorresponding to the wavelength divided by the numerical aperture.21.
 1. In a photomask for optically transferring a lithographic patterncorresponding to an integrated circuit (IC) from said photomask onto asemiconductor substrate, said pattern including a plurality of denselyspaced features and one or more semi-isolated features spaced from thedensely spaced features, said isolated feature having at least one pairof assisting bars on opposite sides of said isolated feature and spacedequally from the corresponding side of the isolated feature opposite theassisting bar, wherein the assisting bars alter the edge intensitygradient of edges of the isolated feature to reduce proximity effects,the improvement comprising: spacing the assisting bars in accordancewith the pitch of the densely space features; and spacing thesemi-isolated features at preferred duty ratios at odd multiples of theduty ratios of the dense features.
 22. The improvement of claim 21wherein the preferred duty ratios include 1:3. 1:5 and 1:7.
 23. In aprojection lithography apparatus with a light source, an illuminationstage for shaping the light source, and a lens for directing the shapedlight onto a photomask for projecting a pattern on the mask to aphotosensitive surface of a semiconductor wafer, a method for locatingoptical proximity correction features on the photomask comprising:providing a set of densely packed mask features on the photomask, saiddensely spaced features having a dense feature pitch; providing one ofmore isolated features on the photomask; locating one or more assistfeatures adjacent the isolated features a distance selected inaccordance with the pitch of the dense features; illuminating thephotomask with off-axis illumination.